The present invention relates to an improved method for preparing high temperature superconductive films and, specifically, to an improved spray pyrolysis method for preparing Bi-Sr-Ca-Cu-O superconducting films.
Spray pyrolysis, a low cost method for preparing thin films, is well known For example, U.S. Pat. No. 4,327,119 to Lis, et al. describes a spray pyrolysis method for synthesizing and producing thin films. The films of Lis, et al. find particular utility in photovoltaic devices, and particularly, in solar heat absorbing panels Similarly, U.S. Pat. No. 4,336,285 to Squillante relates to an improvement on the Lis, et al. method which involves the identification and use of certain highly soluble organic acids as reducing agents in the spray pyrolysis solution. Again, the suggested use is for photovoltaic devices.
Spray pyrolysis has been recognized also as a useful method for applying and preparing superconducting thin films. Maki Kawai, et al. describe the formation of Y-Ba-Cu-O superconducting films using a spray pyrolysis method. Of particular relevance, Hitoshi Nobumasa, et al. describe the formation of Bi(Pb)-Sr-Ca-Cu-O superconducting films using a spray pyrolysis application method. The techniques involve relatively low temperature deposition of chemical precursors, such as metal nitrates and acetates, onto suitable substrates, followed by higher temperature processing, both to convert the precursors to oxides and to induce the formation of the superconducting phase. According to the Nobumasa method, an aqueous solution of Bi, Sr, Ca and Cu nitrates was prepared. A lead doping agent was added as a substitute for a portion of bismuth. The solution was sprayed onto a heated MgO substrate. Thereafter, the sprayed substrate was air-cooled and then reheated to 845.degree. C. After a select heating time of two hours or more, the substrate again was allowed to air cool to room temperature.
While the known pyrolysis methods for applying thin superconductive films have yielded useful high quality films, the films often exhibit high porosity and gross textures. These properties are the result of volume shrinkage and the evolution of gaseous byproducts.
In the literature concerning the processing of bulk superconducting oxides, two methods have been reported for enhancing the densification of Bi-Sr-Ca-Cu-O materials. The first is a melt-quench technique. This method is described by T. Komatsu, et al. in (1) J. Appl. Phys, 27, L533 (1988) and (2) J. Appl Phys. 26, L1148 (1987). The second method is a flux-sinter method in which materials, such as potassium fluoride, are added to the spray solution to enhance mass transport.